1. Field of the Invention
The present invention relates generally to fuel control systems for internal combustion engines and, more particularly, to a cold start fuel control system.
2. Description of the Prior Art
Most modem day internal combustion engines of the type used in automotive vehicles include a plurality of internal combustion chambers. An intake manifold has one end open to ambient air and its other end open to the internal combustion chambers via the engine intake valves. During a warm engine condition, a multi-point fuel injector is associated with each of the internal combustion chambers and provides fuel to the internal combustion chambers. The activation of each multi-point fuel injector is controlled by an electronic control unit (ECU).
During a cold start engine condition, however, a single cold start fuel injector is often times provided in the air intake manifold to the engine. The single cold start fuel injector injects sufficient fuel into the air intake passageway to provide fuel for all of the cylinders of the engine during engine warmup. As the engine warms up, the cold start fuel injector is gradually deactivated while, simultaneously, the multi-point fuel injectors are gradually activated in order to provide a smooth transition between the cold start fuel injector and the multi-point injectors.
In order to ensure engine start up during a cold engine condition, it has also been the previous practice for the cold start fuel injector to inject sufficient fuel into the engine in order to achieve a rich air/fuel mixture having a ratio in the range of 10:1 to 14:1. Even though such a rich air/fuel ratio is sufficient to ensure proper starting of the engine during a cold starting condition, the overly rich air/fuel ratio produces a relatively high amount of undesirable engine emissions such as hydrocarbon and nitrous oxide emissions.
Such an overly rich air/fuel mixture has been required to ensure that there is sufficient fuel vapor within the internal combustion engine in order to ensure engine starting. Such vaporization of fuel is more difficult to attain during a cold start condition than a warm engine condition since the fuel is not vaporized by contacting hot portions, e.g. the internal combustion chamber, of the engine.
While such previously known cold start engine systems have been sufficient to ensure proper starting of the engine while meeting prior governmental regulations, such systems are inadequate to meet the proposed future governmental regulations relating to exhaust emissions from automotive vehicles. For example, the United States emission regulations for CO, HC/NMOG and NO.sub.x for the year 1991 are 7.0, 0.39 and 0.40 grams/mile respectively. For the model year 1997, the corresponding levels must be reduced to 1.7, 0.040 and 0.20 grams/mile, respectively.